Experimental and theoretical study on 3,5-(oxo/thioxo) derivatives of 2,7-dimethyl-1,2,4-triazepines-iodine molecular complexes

摘要

Intermolecular charge-transfer (CT) spectra of 3-thioxo-5-oxo-, 5-thioxo-3-oxo-, and 3,5-dithioxo- derivatives of 2,7-dimethyl-[1,2,4]-triazepine 1:1 molecular complexes with molecular iodine were studied in the UV-visible region. Equilibrium constants and free energy changes of the formed complexes were determined in solution. Ab initio calculations at HF/LANL2DZ* and MP2/LANL2DZ* were carried out to establish the nature of the complexation site, to determine the complex structures, and to examine the basicity of these compounds toward molecular iodine. The 3,5-dithioxo-2,7-dimethyl-[1,2,4]-triazepine is the most basic one toward molecular iodine. In all cases, the complexation takes place at the heteroatom attached to position 3 of the triazepine. Hence, although in general, thiocarbonyls are stronger bases than carbonyls in the gas phase, 5-thioxo-2.7-dimethyl-[1,2,4]-triazepin-3-one behaves as an oxygen base towards I-2. Experimental free energies in solution and gas-phase computational values are linearly correlated.